In this paper, we consider a two-way relay network, where multiple pairs of users exchange their information with the help of a multi-antenna relay station by means of two-way relaying. A relay processing scheme based on minimum mean square error (MMSE) criterion is employed at the relay station to suppress inter- and intra-pair interference. We formulate the optimization problem of minimizing the total transmit power of all users under the relay transmit power constraint and the given sum rate constraint. To solve the optimization problem and get the joint relay processing and power control policy, we propose an iteration algorithm. Simulation results show the performance of the proposed schemes and are coherent with what we expected.